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Standard · IEC 61025:2006
IEC 61025 — Fault Tree Analysis
IEC 61025:2006 Fault Tree Analysis (FTA) is the international standard that fixes the symbols, semantics and quantification rules for fault trees. Every other safety standard that calls for FTA — ISO 26262, ARP 4761, EN 50126, IEC 61511, MIL-STD-882 — points back to IEC 61025 for the underlying technique.
What IEC 61025 actually requires
The standard is short and prescriptive. It defines:
- Symbol library (Annex A) — exact geometry for AND, OR, k-out-of-n (voting), INHIBIT, NOT/exclusive-OR, priority-AND, transfer in/out, basic event, undeveloped event, conditioning event, house event.
- Top-down deductive method — start from the undesired top event, decompose only as far as the analyst's resolution, never invent intermediate events to make the picture neater.
- Boolean algebra of the tree — the tree is read as a Boolean expression mapping basic-event states to the top-event state.
- Cut-set determination — minimal cut sets (the smallest combinations of basic events whose joint occurrence causes the top event), typically computed via MOCUS or BDD.
- Quantitative evaluation — compute top-event probability either exactly (rare-event approximation, inclusion-exclusion) or by Monte Carlo over the basic-event distributions.
- Importance measures — Birnbaum, Fussell-Vesely, RAW (risk achievement worth), RRW (risk reduction worth), criticality.
How FTA Studio implements each clause
| IEC 61025 clause | FTA Studio feature |
|---|---|
| Annex A (symbol library) | All eleven symbol classes drawn to specified geometry — gates, basic/undeveloped/house events, conditioning events, transfer in/out. |
| §5 (deductive construction) | Top-event-first builder with multi-page tree support and gate-typed children. |
| §6 (qualitative analysis) | MOCUS minimal cut-set algorithm with deterministic, repeatable output. |
| §7 (quantitative analysis) | Exact Boolean evaluation, rare-event approximation, and (Enterprise) Monte Carlo over lognormal/uniform/triangular leaf distributions. |
| §7.4 (importance measures) | Birnbaum, Fussell-Vesely, RAW, RRW, criticality — exported to CSV alongside the tree. |
| §8 (CCF treatment) | Beta-factor and Multiple Greek Letter (MGL) common-cause models attachable to gates. |
| Annex C (interchange) | IEC-format JSON export — round-trippable, schema-validated, suitable for archival. |
Standards that reference IEC 61025
FTA Studio's templates illustrate IEC 61025 in the contexts where it gets cited:
- ISO 26262 (automotive functional safety) — calls for FTA at ASIL-C and ASIL-D.
- ARP 4761 (aerospace) — FTA is the deductive technique for the SSA.
- EN 50126 / EN 50128 / EN 50129 (rail RAMS) — FTA at SIL 3 and SIL 4.
- MIL-STD-882E (defence system safety) — FTA in the system safety program plan.
- IEC 61508 / IEC 61511 (functional safety, process) — FTA in the SIS verification.